Slurries or sludges which require dewatering may be inorganic or organic, natural or manmade, fibrous or non-fibrous. Slurries usually require some form of chemical, thermal or biological pretreatment to enhance their dewaterability. Subsequent processes after dewatering usually depend upon the dryness of the solids or other characteristics of the cake to be efficient. It is therefore paramount that the dewatering process be properly designed and optimized. When optimizing a belt filter, the major considerations are cake dryness and mass throughput rate. The efficiency of a belt press is therefore a function of quantitative cake characteristics, as well as, qualitative parameters, such as ability to release and be transported from the belt.
While the terms water, dewater and sludge are used herein, it will be recognized that these terms are interchangeable in concept with the words liquid, concentrate and solids.
The use of horizontal belt filters to remove liquids, such as water, from slurries containing solids is well known and widely practiced. It also is known to cover and press the slurry on the belt filter with a second belt to form what is conventionally known as a belt filter press.
A belt filter press is usually employed as part of a continuously operated process for removing liquids from solids. In the most general case, the process equipment consists of a continuous flow conditioner where coagulant aids, such as polyelectrolytes, are introduced into the slurry; a mixing unit where the coagulant aid is contacted with the slurry to coagulate the solids; a gravity drainage zone where liquid is drained by gravity from the solids on a filter belt to form a solids cake; and a press zone where a second moving belt covers the solids cake and the pair of belts, with the solids cake between them, are pressed between rollers to deform the solids cake and remove additional liquid. The solids cake emerging from the press zone is then removed from the belts, usually with the aid of a scraper.
In the press zone, the solids cake is subjected to a variety of forces and pressures including radial shear, flexing or bending, and pressures perpendicular to the face of the rollers. In addition, as the solids cake passes through the rollers, it is alternately exposed to pressures of high and low magnitude. These forces and pressures, which include bending, shear and normal forces and the freedom of the sludge to migrate laterally on the belt, create the conditions by which the liquid is removed from the solids cake. Deformations of the cake around a roller creates microchannels for water to flow from within the cake to the surface of the cake where it is removed by passing through the belt. The faces of the belts also deform the solids cake while it is passing around the rollers, which aids in the capillary removal of liquid from the surface of the cake. It is noteworthy that in the press zone, the sludge is free to move laterally between the belts (known as migration) and also to penetrate into the weave of the filters (known as extrusion). The migration is advantageous in that sludge becomes exposed to new belt face for water to flow, but has the disadvantage that large amounts of migration ultimately demand a lowering of the mass loading rate a machine can effectively dewater. Failure to account for area migration will result in sludge oozing out the edges of the belts which is inefficient and also causes housekeeping problems. The degree to which a sludge penetrates the weave of the filter determines the release characteristics of the sludge, and the cleanliness of the filter materials, both of these factors affect the efficacy of the pressing operation.
The complex interrelations between the coagulant aid(s) employed, the solids and liquids of the slurries, the physical properties of the filter belt materials, the belt speed, roller dynamics and belt tensioning makes a priori knowledge of the optimum loading rates difficult to obtain. Therefore, when considering the types of data needed to design or to optimize the dewatering process on a particular apparatus, it is desirable to have detailed knowledge of the nature of the slurry, the quantity and type of coagulant aid required, the amount of slurry which can be placed on a unit area of the filter belt per unit time, the amount of liquid removed in the gravity drainage zone, the amount of liquid removed in the pressure zone, the area expansion or migration of the sludge cake, and a qualitative estimate of the capacity of the cake to be released by the belt.
The present methods of obtaining data for design of a full scale process usually involves running the process with the various proposed operational, loading, or conditioning changes on the smallest available model of the full sized apparatus. Operational optimization of existing units usually requires that a running machine be tested at the proposed process conditions. This results in downtime and other undesirable operational problems. It is therefore not practical or economically feasible to gather data by pilot or full scale testing.
In a review of the design and sizing of sludge dewatering equipment in 1978 by Campbell, Rush, and Tew, the authors reported "very little information is reported in the literature on the methodology used to determine full scale requirements . . . " and "bench tests are not available for the generation of design data . . . Pilot scale machines used to test a sludge are usually the smallest full scale machine made by a manufacturer." The state-of-the-art for collecting belt filter design data has not advanced significantly. The Buchner funnel test and the filter leaf test, both designed for vacuum filter simulation, are still being used for belt filter simulation, indicating a persistent need to develop an improved test method and apparatus.
There are currently four methods of testing slurry dewaterability. The first two tests, the Buchner funnel test and the filter leaf test, utilize vacuum pressure across a stationary filter to dewater the slurry or sludge. While useful for testing the application of vacuum filters, these tests do not properly recreate the physical nature of a belt filter press. A third method for characterizing dewaterability is the capillary suction test. The capillary suction test measures the movement of water from a sludge on a blotter paper by capillary action. The rate of mobility of water is measured with conductivity probes. The relationship between the measured rate and the ability to dewater a sludge on a belt filter press is not clear. Pressure cylinders are the fourth type of test available. In these devices, sludge is squeezed at known pressures using a pressure cylinder. These devices fail to adequately represent belt filter press operation because they do not allow for shear stress or migration of these sludges on the filter cloth and the sludge cake or the belt filter deformed. Deformation of the cake and the belt are important in the dewatering process, for these parameters are related to the ability of the cake to release from the belt. Furthermore, the pressure forces applied in a cylindrical test unit are not applied or released with the rapidity which occurs on a full scale belt filter unit. The cycling of forces with time and the differential movement of the two belts with respect to the sludge cake during the filter belt press process create the conditions of pressure, shear and capillary action which cause the sludge to release water.
The removal of liquids, such as water, from a slurry, such as sludge, by a belt filter press is a complex process. Therefore, it is desirable to have a laboratory scale prototype of a belt press apparatus which adequately simulates the complex forces which act to remove liquid in a full scale belt filter press.
There also is an interest and a need to improve the efficiency of the belt filter press process of dewatering slurries. As a result there are a number of patents on improvements in belt filter press equipment.
The Pierson U.S. Pat. Nos. 4,446,023 and 4,472,779 disclose the use of pistons or levels in a belt filter press to move a continuous belt in a stepwise fashion in an effort to improve efficiency.
The Bratten U.S. Pat. No. 4,568,460 discloses a dual pressing system where a continuous belt press is augmented by passing the belt between externally driven pressure plates. The pressure plates are included to squeeze excess water from the cake.
The Pietzsch U.S. Pat. No. 4,861,495 discloses a system which includes a rotating drum within a loop of a moving belt filter cloth to increase shear on the sludge and to keep the belt tension constant and to thus more effectively remove water.
A need still exists for additional improvements which increase the dewatering efficiency of belt filter press equipment. The belt press as currently available on the market is a continuous service apparatus which is constantly fed conditioned slurries and which constantly discharges dewatered cakes and filtrates. The present invention instructs the practitioner how a batch mode filter may be constructed that allows off-line optimization of operational parameters, such as coagulant and addition rate, type of belt material and mass feed rate.